This invention relates to a process for catalytically reacting gases having a high SO.sub.2 content in contacting plants having a plurality of series-connected contacting trays, wherein a partial stream of partly reacted gases containing SO.sub.3 is admixed with the SO.sub.2 -containing gas before entering the first contacting tray, and the gases are subjected to interstage cooling between contacting trays.
During the reaction of SO.sub.2 -containing gases to form SO.sub.3, which is subsequently used to produce sulfuric acid, the catalyst must be heated first by the gas to the so-called initiation temperature. This initiation temperature depends on the composition of the catalyst and on the process by which it has been made and, e.g., with catalysts based on vanadium pentoxide (V.sub.2 O.sub.5) is about 400.degree.-450.degree. C. The reaction of SO.sub.2 to form SO.sub.3 results in a temperature rise because the reaction is exothermic. With gases which contain up to about 11% SO.sub.2, the reaction is arrested at temperatures of about 620.degree. C., where the equilibrium of the reaction SO.sub.2 + 1/2 O.sub.2 .revreaction. SO.sub.3 is reached. With gases having a higher SO.sub.2 content, the temperature continues to rise because the reaction reaches its equilibrium only at higher temperatures. However, the catalyst will deteriorate at temperatures above about 620.degree. C.
To avoid overheating which deteriorates the catalyst, SO.sub.3 -containing gases have been admixed with the SO.sub.2 -containing gases before entering the first contacting tray. The admixed SO.sub.3 retards the conversion of the SO.sub.2 to SO.sub.3 so that overheating will be avoided if the admixing and the residence time of the gases in the contacting trays are properly controlled (DAS 1,054,431, German Patent 504,635, DOS 2,026,818).
Because the catalyst loses activity in the course of time, particularly in the first contacting trays, the rate of the reaction SO.sub.2 + 1/2 O.sub.2 .revreaction. SO.sub.3 and the conversion of the gas to form SO.sub.3 are reduced accordingly. In the treatment of gases containing less than about 11% SO.sub.2, more catalyst may be used than is theoretically required. This is not possible, however, in the case of gases having a high SO.sub.2 content (e.g. more than 11% SO.sub.2) because the reaction must be interrupted before the temperature which is critical for the catalyst is reached, i.e., considerably before the equilibrium is reached (cf. U.S. pat. No. 3,875,294 issued Apr. 1, 1975.)